1. Field of the Invention
The present invention relates to a method for inhibiting significant oxidation of a film on a substrate. According to one embodiment, this invention relates to a method of inhibiting significant oxidation of films of electrochromic materials comprising a non-stoichiometric, oxygen deficient metal oxide film provided by pyrolytic deposition techniques wherein the non-stoichiometry of the metal oxide film on the glass is maintained during heating and bending of the glass.
2. Discussion of the Related Art
Electrochromic devices are devices in which a physical/chemical change produced in response to the induced electrical field results in a change in the reflective (or transmissive properties) of the device with respect to electromagnetic radiations, e.g., UV, IR and visible radiation. Such devices generally comprise a film of electrochromic material. The electrochromic film usually comprises an inorganic metal oxide most commonly a transition metal oxide, in particular, tungsten oxide film; more particularly non-stoichiometric tungsten oxide film provided according to the teachings of U.S. Pat. No. 4,960,324, assigned to Ford Motor Company.
One of the problems solved by the teachings in U.S. Pat. No. 4,960,324 is that it teaches a method which is suitable for coating large areas such as would be necessary if, e.g., sunroofs or windshields of automobiles were to be made as electrochromic devices. It also teaches a sequential process suitable for providing multiple layers of the various films required for an electrochromic device. As would be apparent, it would be advantageous to make sunroofs or windshields electrochromic devices which could be colored to desired intensity to keep out radiation like UV, IR and visible transmissions at will. For example, it might be desirable to "color" the sunroof and the windows to allow minimum transmittance when the automobile is parked to prevent the interior of the automobile from heating up on a sunny day. In another embodiment, the windshield might be colored to an intensity which allows operation of the automobile yet reduces the amount of visible, UV, and IR transmission through the windshield.
Non-stoichiometric, oxygen deficient tungsten oxide films have been observed to show excellent electrochromism. Electrochromism is also known to occur in several transition metal oxides. The characteristics of electrochromism are manifested by a reversible color change, usually switching from an uncolored state to a colored state, or vice versa, as a result of an applied electric current. From a building energy efficiency viewpoint in regards to using electrochromic glazings, the ability to dynamically control incoming solar radiation either in the visible or near infrared spectral regions is very attractive. Electrochromism also has an important future in automotive glazings. The properties of certain non-stoichiometric metal oxide films, more particularly non-stoichiometric, oxygen deficient tungsten oxide film is very important to the development of large area optical shutters which may be used as information displays or as windows for motor vehicles.
However, the glass which serves as a support for such electrochromic non-stoichiometric metal oxide films is usually subjected to a bending process to bend the glass part to conform with the shape and form of automotive glazing. In the event that the non-stoichiometric, oxygen deficient metal oxide film is to be provided on the glass support prior to bending, the subsequent bending of the glass/metal oxide film system leads to a diminished electrochromic character of the non-stoichiometric oxygen deficient metal oxide film. It is believed that since the bending process is normally performed in air at temperatures of about 1200.degree. F., the diminished electrochromic character of the non-stoichiometric metal oxide film is caused by the oxidation of a non-stoichiometric metal oxide film before bending, to a stoichiometric metal oxide film after bending the glass support. Present methods for providing the electrochromic layer are incapable of providing a non-stoichiometric metal oxide layer which can then be subjected to the bending and fabrication processes required to transform the non-stoichiometric metal oxide layer on glass to the required shapes and sizes for sunroofs or windshields without structurally changing the layer.
Another problem encountered with the electrochromic non-stoichiometric metal oxide layer is that when the layer is subjected to glass bending processes which comprise heating and bending, the layer becomes stoichiometric because of oxidation and therefore the layer loses its electrochromic efficiency. This is particularly problematic if the metal oxide layer is to be used in a device that requires many cycles to keep out undesirable radiation, as would be intended by a sunroof or windshield of an automotive vehicle or a window of a building.
Still another problem of such metal oxide layers is the introduction of micro cracks in the layer due to film stress when the layer oxidizes from a non-stoichiometric metal oxide layer to a stoichiometric metal oxide layer during the bending and fabrication process. These micro cracks in the metal oxide layer are undesirable because they are partly responsible for degradation of the layer.
It is desirable to have a method which could maintain the sub-stoichiometry of the electrochromic metal oxide film on glass during bending of the glass. This would help guarantee that such a metal oxide layer, when used in an electrochromic device, is capable of switching for a prolonged period of time without significantly changing its electrochromic activity. It would also be advantageous if the method for maintaining the electrochromic metal oxide sub-stoichiometry would be simple and commercially suitable for applying to large areas easily.